Deuterium isotope effects, chemical shifts types

For the pyridine complexes, effects due to complexation observed at the pyridine molecule (such as N chemical shifts) can also be used . Low temperature measurements have clearly been very useful in elucidating these reactions. An approach using N and H chemical shifts as well as deuterium isotope effects on N chemical shifts and primary proton isotope effects (see Section n.F.7) at very low temperature in freons showed in the N spectrum three different species AHB, AHAHB and AHAHAHB. For the 1 1 complex an asymmetric single well potential is assumed , different from the approach taken above. Furthermore, a linear correlation was found between the N chemical shift and the one-bond 7(N,H) coupling constant. This type of reaction has also been studied using fractionation factors (See Section n.O). 

Schiff bases are important molecules also in biological contexts. Schifif bases of ortho-hydroxy type (Figure 6.26) represent both static and tautomeric cases. Primary deuterium isotope effects are small in a static case. For tautomeric systems, the primary deuterium isotope effects on chemical shifts are temperature dependent [50]. This was also investigated in detail for a series of Schiff bases (X = 4-methoxy, 5-methoxy, and 4,6-dimethoxyphenyl of Figure 6.26). These have been studied in detail using secondary isotope effects on chemical shifts. One approach is the simple one presented in Eq. (6.1). The other one is based on the Limbach approach [40]. The presence of an equilibrium is established in many of these Schiff bases. A... 

A different way is to estabhsh the sign of the deuterium isotope effects on chemical shifts for different types of tautomers and equilibria as seen in Figure 3.15. 

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